A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
In order to image the pattern via the lens to the substrate, the layer of resist provided on the substrate should be in the focal plane of the projection system. Focus tests have been developed to test if the substrate is positioned correctly in which a test pattern provided by a test device is imaged on the layer of resist. Next, a latent image of the test pattern is made visible by performing, for instance, a post exposure bake. After this, the width of, for instance, a line of the created pattern could be measured using, for instance, a scanning electron microscope. By comparing this width with a previously obtained calibration graph (bossung curve) the defocus can be determined. It will be understood that the width of a line is minimal in the best focus position and will become larger with increasing defocus.
In some places this text refers to positioning the substrate in the focal plane of the projection system. It is understood that this should be read as positioning the layer of resist provided on the substrate in the focal plane of the projection system.
However, in telecentric focus tests, it is only possible to determine the absolute defocus, but not the relative defocus. The absolute defocus is the distance between the layer of resist and the focal plane, but does not provide information whether the resist is above or below the focal plane, i.e. the sign of the defocus can not be determined. The relative defocus is the distance between the layer of resist and the focal plane including the sign of the defocus. The relative defocus also provides information about whether the resist is above or below the focal plane. Telecentric focus tests only provide information about the absolute defocus and do not provide sufficient information for correcting the position of a substrate with respect to the projection system, since the sign of the absolute defocus is not known.
To overcome this problem, tilted focus tests have been developed. In such tilted focus tests, an image of the test pattern is projected under an angle, such that defocus not only results in a blurred image of a test device, but also in a lateral shift of the position of the imaged test device. Such tilted focus tests use devices to create a tilted beam, such as a wedge that is translucent for the light used, or a transmissive test device provided with a pattern that creates a diffraction pattern in which e.g. the minus one diffraction order is cancelled out. These state of the art techniques are discussed in more detail below, with reference to FIGS. 2a, 2b, 3a, 3b, 4 and 5.
In order to manufacture increasingly smaller lithographic devices having increasingly smaller dimensions, the wavelength of the radiation used is preferably chosen as small as possible. The use of extreme ultraviolet radiation, e.g. in a range of 5-20 nm, would therefore offer great advantages. It is known, however, that radiation having a relatively small wavelength, such as EUV radiation, does not easily penetrate through matter. Therefore, the prior art solutions described above and below with reference to FIGS. 2a, 2b, 3a, 3b, 4 and 5 to create a tilted beam cannot be used effectively for EUV radiation. In principal, it is possible to manufacture a reflective version of the known transmissive test devices provided with a pattern. However, this requires an accuracy that is difficult to obtain these days, as will be further explained below.
US2002/0015158 A1 discloses a method of detecting focus information based on illumination rays having different main ray incidence directions, which means that the projection beam is tilted. Images of marks are projected through an optical system. A blocking member is provided in the illumination system that can be positioned in the light beam. The blocking member is provided with an aperture to partially block the light beam in such a way that a tilted beam is generated.
In order to determine the lateral shift of the projected mark images with respect to a mark image projected using a non tilted beam, a reference is needed. According to US2002/0015158 A1, this is achieved by superimposing a first and a second image of a single mark on the reticle. The mark is first projected on a substrate using a tilted beam, and next, another projection on the substrate using a non tilted beam is superimposed on the previous one. In between the first and second projections, the blocking member is removed from the light path. The lateral shift is given by the mutual distance between the first and second projected marks on the substrate.
According to a further embodiment of US2002/0015158 A1, a first exposure is carried out using a first blocking member, and a second exposure is carried out using a second blocking member, where the first and second blocking members have an aperture that are opposite to each other, such that the tilts of the first and second exposure are opposite to each other and thus the sensitivity of the measurement method is doubled.
The tilted focus test as disclosed in US2002/0015158 A1 has however, some disadvantages. First, the blocking member must be specially designed to take into account the relative position of the marks on the reticle. This means that a blocking member needs to be designed with knowledge of the relative position of the mark on the reticle.
Second, according to US2002/0015158 A1 double exposure is needed to provide a reference, in between which the blocking member needs to be removed or replaced with another (opposite) blocking member. Therefore, this is a time-consuming procedure that reduces the throughput of the system.
US2002/0100012 A1 describes several ways to create a tilted beam for use in a tilted focus test. A tilted beam is obtained by blocking certain diffraction orders. The blocking can for instance be achieved by positioning a pellicle under the mask. A frame member holding the pellicle is then used to block a diffraction order of the mark. According to an alternative, part of the normally transparent part of the pellicle can be non-transparent, to block certain diffraction orders.
The options presented in US2002/0100012 have several disadvantages. Using the frame member to create a tilted beam is rather cumbersome. Additionally, this method is used if the angle under which the diffraction orders are emitted by the mark and the distance between the frame member and the mark are within certain limits with respect to each other. Using the normally translucent pellicle to block certain diffraction orders requires a specially adapted pellicle for each mark. The overall performance of the pellicle will decrease, as a result of the non-transparent parts, having a negative effect on the over-all performance of the system. Also, pellicles can not be used in applications using (extreme) ultraviolet radiation beams, as will be understood by a skilled person.